The dispensing of medication via the respiratory system is receiving increasing use. Medications are commonly in the form of aerosols or powders obtained from pressurized containers or generated by atomizers. The dispensing methods used include patient operated hand-held respirators. To be effective the medication must be released in coordination with inhalation. Often the patient operates a control or actuator to release medication during inhalation.
In cases where coordinating the release of medication with inhalation is difficult or must be precisely controlled, an inhalation sensing mechanism is preferred to initiate the medication release. In U.S. Pat. No. 6,354,290 B1 Howlett describes a means to initiate the medication release with a vane that is deflected by the flow of the user's breath. Variations of this approach can be found in several patents. This type of respirator typically includes an arming or cocking control that the user manually activates to allow a subsequent inhalation to initiate the release of medication. The control is then manually reset for the next use. Consequently, this method is primarily used with medications that are administered in a single inhalation.
Extended Dosage
The dosage rate limit for drugs such as tobramycin (TOBI) is such that they must be applied over an extended period of breathing. Depending on the size and health of the patient, delivery of a dose will require 100 to 150 inhalations. Other drugs in either aerosol or powder form may deviate from this typical range. Manually activating the release of medication in coordination with inhalation for such a large number of steps is difficult for an active adult and virtually impossible for a pediatric or severely ill patient. The difficulty of this process is compounded if it is necessary to remove the respirator from the mouth or nose for each exhalation.
Respirator dispensation of drugs through extended breathing is substantially improved by automatically controlling medication release with an inhalation detector and permitting tidal breathing. That is, both inhale and exhale through the respirator. This seemingly simple change places an added set of requirements on the respirator. During a protracted breathing regimen a person with a serious lung ailment, a common user of such a device, has a very limited tolerance for resistance to air flow. Consequently, the air path through the respirator, including the inhalation detector, must have minimal pressure drop.
Tidal breathing and coughing may result in a backflow of moisture, medication, and sputum that coats internal respirator surfaces. Respirator elements that are exposed to this backflow must function with a coating of material that is often viscous, conductive, and corrosive. Because these backflow deposits can be highly infectious, the air passages, inhalation detector, and medication release device must either be disposable or regularly immersed in cleaning solutions. A typical cleaning process will consist of rinsing with agitated soapy water and/or a mild acid solution followed by flushing with running water. For cleaning processes to be effective the portion of the respirator exposed to contamination must be accessible to cleaning processes and free of crevices, holes, cavities, or absorbent surfaces.
Preferably, a dispensing respirator distributed through a pharmacy or medical practitioner for patient use will not require adjustment or calibration for or by the individual patient. The range of respiration flow rates such a respirator must accommodate is quite wide. The rate of a robust adult can be 10 to 12 times that of a child. Normal respiration flow rates fall between 7 and 80 liters per minute.
Requirements
The sensing of respiration is primarily a task of detecting the direction and magnitude of gas flow, usually air. The existing art of measuring gas flow has a long history and includes a wide range of processes. The task of sensing tidal respiration, particularly as applied to dispensing medication to the diseased or impaired, encounters several restrictions and demands that, in the aggregate, are not met by prior art.
The key requirements for an inhalation detector used in a tidal breathing respirator are:    (a) Operate at a pressure drop of no more than 0.04 cm H2O at a flow rate of 7 liters per minute.    (b) Function over a flow range of 7 to 80 liters per minute without adjustment.    (c) Tolerate abrupt surges such as encountered in coughing.    (d) Sense flow direction to discriminate between inhalation and exhalation.    (e) Properly operate after the backflow of moisture, sputum, and medication have coated mechanisms and surfaces.    (f) Tolerate regular flushing and cleaning with disinfectants.    (g) Configured to be free of crevices, cavities, or absorbent surfaces that may trap infectious residue.    (h) Operate at a power level suitable for battery operation.    (i) Have a low cost.